1. Field of the Invention
The invention relates to a frequency synthesizer and more particularly to a frequency synthesizer in a multi-band orthogonal frequency division multiplexing (MB-OFDM) ultra wideband (UWB) system.
2. Description of the Related Art
The frequency band of a multi-band orthogonal frequency division multiplexing (MB-OFDM) ultra wideband (UWB) system, from about 3.1 to 10.6 GHz, is divided into 14 bands, each having a bandwidth of 528 MHz. FIG. 1 is a band diagram for a MB-OFDM UWB system. As shown, central frequencies of the 14 bands, referred to as first to fourteenth frequencies f1 to f14 from left to right, are respectively 3432 MHz, 3960 MHz, 4488 MHz, 5016 MHz, 5544 MHz, 6072 MHz, 6600 MHz, 7128 MHz, 7656 MHz, 8184 MHz, 8712 MHz, 9240 MHz, 9768 MHz, 10296 MHz, with any adjacent two frequencies separated by a base frequency fdm (528 MHz).
MB-OFDM UWB systems require that the frequency synthesizer generating the 14 frequencies has high band switching speed. Typically the switching time must be less than 9.5 ns. Several UWB frequency synthesizers have been proposed to generate a band within 3˜8 G. The following are representatives among them: [1] J. Lee, and D. W. Chiu, “A 7-Band 3-8 GHz frequency synthesizer with 1 ns band-switching time in 0.18 um CMOS technology,” ISSCC Dig of Tech. Papers, pp. 204-205, February 2005, [2]: C. C. Lin, and C. K Wang, “A regenerative semi-dynamic frequency divider for mode-1 MB-OFDM UWB hopping carrier generation,” ISSCC Dig of Tech. Papers, pp. 206-207, February 2005, [3]: A. Ismail, and A. Abidi, “A 3.1 to 8.2 GHz direct conversion receiver for MB-OFDM UWB communications,” ISSCC Dig of Tech. Papers, pp. 206-207, February 2005, and [4]: D. Leenaerts et al., “A SiGe BiCMOS Ins frequency hopping frequency synthesizer for UWB radio,” ISSCC Dig of Tech. Papers, pp. 202-203, February 2005.
None of the UWB frequency synthesizers is capable of generating all 14 frequencies. The reason is that many phase locked loops and mixers are required to cover such a wide band, requiring larger chip area, high power consumption, and excessive spur energy. Additionally, it is difficult to reduce the switching time to less than 9.5 ns.
High-speed wireless transmission is an unavoidable trend in communication development. UWB apparatuses comprise functions in addition to communication. Accordingly, a MB-OFDM UWB system covering the first to fourteenth frequencies, having high switching speed and moderate or low spur energy is desirable.